Hook up ui::Compositor to Display's BeginFrameSource

cc/surfaces/display_scheduler.cc

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/surfaces/display_scheduler.h"
 
 #include <vector>
 
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
@@ skipping 16 matching lines @@
       all_active_child_surfaces_ready_to_draw_(false),
       pending_swaps_(0),
       max_pending_swaps_(max_pending_swaps),
       observing_begin_frame_source_(false),
       root_surface_damaged_(false),
       expect_damage_from_root_surface_(false),
       weak_ptr_factory_(this) {
   begin_frame_deadline_closure_ = base::Bind(
       &DisplayScheduler::OnBeginFrameDeadline, weak_ptr_factory_.GetWeakPtr());
 
-  // TODO(tansell): Set this to something useful.
+  // TODO(enne): remove this once the renderer uses begin frames everywhere.
   begin_frame_source_for_children_ = SyntheticBeginFrameSource::Create(
       task_runner, BeginFrameArgs::DefaultInterval());
+
+  // TODO(enne): For now, this is always the same, so just set initially.
+  client_->UpdateSchedulerBeginFrameSource(begin_frame_source_);

[enne (OOO), 2016/03/25 17:55:36]

I'm not sure about the split between DisplayScheduler and Display in terms of
registering BeginFrameSources.  Right now I left it as the DisplayScheduler
picking the top level BFS it wants to use (which could be unthrottled, and isn't
in this patch yet), and then the Display registering it with the current surface
id and handling surface id changes (which I don't think happen, but that's not
asserted anywhere).

 }
 
 DisplayScheduler::~DisplayScheduler() {
   if (observing_begin_frame_source_)
     begin_frame_source_->RemoveObserver(this);
 }
 
 // If we try to draw when the root surface resources are locked, the
 // draw will fail.
 void DisplayScheduler::SetRootSurfaceResourcesLocked(bool locked) {
@@ skipping 253 matching lines @@
 }
 
 void DisplayScheduler::DidSwapBuffersComplete() {
   pending_swaps_--;
   TRACE_EVENT_ASYNC_END1("cc", "DisplayScheduler:pending_swaps", this,
                          "pending_frames", pending_swaps_);
   ScheduleBeginFrameDeadline();
 }
 
 }  // namespace cc